Re: basic-block and profile-based optimizing (was Re: New attribute"infrequent"?)

[Scott A Crosby <crosby at qwes dot math dot cmu dot edu>]

On Fri, 31 Aug 2001, Andreas Jaeger wrote:

> David Edelsohn <dje@watson.ibm.com> writes:
>
> >>>>>> Andreas Jaeger writes:
>
> I forgot the following (Honza stated it in one of his first emails) -
> and this is IMO the key issue:
>
> If GCC notices that a function is infrequently used, e.g. only used for
> error handling, and normally not used, a call to this function can be
> placed out of line, a branch to a block that contains such a call can
> be predicted as not taken.  But how does an application get the
> information that a function of the library is infrequently used?

This doesn't really matter, because the application itself presumabely
will be going through profile-based optimization.. Thus, when profiling
the application, we'll notice that the function call is seldom taken, and
thus we'll code it out-of-line at that time.

I don't think that adding such an attribute-mechanism to a library
interface is all that necessary, nor will it be worth the work and
maintance.

In addition, this may be a pessimization.. If an infrequently called
library function is invoked from an application function, we only want to
code it out-of-line if the application function has a lot of invocations,
which we won't know till profile-based feedback. Otherwise, doing it out
of line could just gives size and complexity.

Scott


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